Until recently, most one-way pager users either had service in a single simulcast area or had nationwide service. For the first type of service, messages are simulcast from a single group of transmitters. For nationwide service the messages are replicated on all simulcast areas across the nation. Obviously, nationwide service is very expensive for the service providers.
Recently, "Follow Me Roaming" (FMR) has become available. Under FMR, the user normally has service in only one simulcast area, i.e. the user's "home" area. When the user plans to travel outside the home area, he notifies the service provider of his planned itinerary, including departure and arrival times so that his messages can follow him.
FMR is becoming the method of choice by the service providers for providing nationwide coverage, because messages do not have to be sent in all coverage areas. They just have to be sent to the area where the user is. The most thoughtfully implemented FMR systems operate with an "overlap interval" around the departure and arrival times. During the overlap interval, messages are replicated both in the area being departed and in the arrival area. This is done to provide a "grace period" having upper and lower limits for accommodating temporal variations in the departure and arrival times. Additional time generally is put into the upper limit of the overlap interval to accommodate uncertainty in the channel search time required for the pager to find a new messaging system in each arrival area.
Uncertainty in the channel search time occurs because prior art subscriber units do not search for a new system until they lose the signal from a currently received system. Because system coverage areas can overlap, it is possible for a subscriber unit that has visited, for example, a second system adjacent to the home system to remain locked to the second system for a long time--possibly hours or days--after the subscriber unit has returned to the home area. If message replication is halted before the subscriber unit has switched back to the home area, messages transmitted only in the home area will be missed. On the other hand, message replication costs air time, causing the service provider to want to keep message replication time as short as possible.
A related problem affects a prior art two-way subscriber unit which has switched to one-way operation with a one-way system, due to the unavailability of a usable two-way system. If the subscriber unit remains within range of the one-way system, it may not search for a more appropriate two-way system for a long time, even when a usable two-way system becomes available.
Thus, what is needed is a method and apparatus for providing a predictable upper bound for a channel search time during which a portable subscriber unit searches for a more appropriate system for its message delivery. The method and apparatus preferably will allow the portable subscriber unit to find the more appropriate system even when monitoring a currently selected system, and without missing any messages.